Phosphate rock contains fluorine compounds, such as fluorapatite. Beneficiated phosphate rock from Florida typically contains 3 to 4% fluorine values. Phosphoric acid can be produced from this rock by making a slurry of the rock in phosphoric acid and reacting the slurry with sulfuric acid. Part of the fluorine present in the rock is evolved as silicon tetrafluoride and gaseous hydrofluoric acid, which upon scrubbing with pond water, form a dilute fluosilicic acid. For example, U.S. Pat. Nos. 3,091,513 and 3,273,713 disclose such scrubbing. By efficient design of scrubbers, it is possible to continuously produce a solution containing about 20% fluosilicic acid. The acid solution usually contains impurities, including 1,000 to 4,000 parts per million of P.sub.2 O.sub.5.
Although the solution is useful per se, as for treating municipal drinking water, it is sometimes desirable to recover the fluorine values present in the fluosilicic acid as fluoride salts and/or as anhydrous hydrofluoric acid.
Many processes have been developed for concentration of dilute fluosilicic acid solutions, and preparation of hydrofluoric acid from the concentrated fluosilicic acid. Such attempts are described in U.S. Pat. Nos. 3,645,678; 3,645,679; 3,689,216; 3,855,399; 3,278,265; 3,218,124; 3,256,061; 3,140,152; 3,914,398; 3,537,817; 3,758,674; German Offen. Nos. 2,035,300, 2,032,855, and 2,248,149; and French Pat. No. 7,034,470. However, these processes suffer from one or more disadvantages. Disadvantages of these processes include operation at excessively high temperatures or under severe conditions, use of an excessive number of processing steps, consumption of uneconomical quantities of raw materials, production of undesirable by-products, production of contaminated hydrogen fluoride, low yield of hydrogen fluoride, and considerable expenditure, both in terms of operating expense and initial capital investment. For example, U.S. Pat. Nos. 3,218,124 and 3,689,216 describe a process where fluosilicic acid solutions are treated with concentrated sulfuric acid to liberate silicon tetrafluoride and hydrogen fluoride, which are then separated. The silicon tetrafluoride is hydrolyzed to fluosilicic acid which is recycled and SiO.sub.2 which is removed. Two disadvantages of this process are that a large volume of concentrated sulfuric acid is required per unit of fluosilicic acid and the splitting of the fluosilicic acid must be carried out at relatively high temperatures. This can result in severe corrosion of equipment.
U.S. Pat. No. 3,256,061 describes a process whereby fluosilicic acid is neutralized with ammonia, producing ammonium fluoride and silica. The silica is separated by filtration, and the ammonium fluoride is concentrated to a molten state constituting NH.sub.4 F--NH.sub.4 HF.sub.2, which when treated with concentrated sulfuric acid produces hydrogen fluoride. Ammonia remains in the sulfuric acid and is sent to a phosphate acidulation unit. The chief drawbacks of this process are the requirement to recycle ammonia and the failure to remove any P.sub.2 O.sub.5 impurity in the fluosilicic acid. A similar process is described in U.S. Pat. Nos. 3,914,398 and 3,537,817. U.S. Pat. No. 3,549,317 discloses reacting fluosilicic acid with calcium carbonate at a temperature of at least 50.degree. C. to form calcium fluoride and silica. The hot CaF and silica-containing reaction mixture is then reacted with sodium hydroxide to form a calcium fluoride precipitate and a silicate solution (from which silica can be precipitated at above about 50.degree. C. by diluting with an acid to a pH between 7 and 9. U.S. Pat. No. 3,000,702 discloses reacting fluosilicic acid at 50.degree. C. to 90.degree. C. with anhydrous ammonia to produce a solution of ammonium fluoride and a filterable precipitate of silica. Japan Pat. No. 46-20134 to Kureha relates to reacting sodium fluoride with sulfuric acid. U.S. Pat. No. 3,878,294 describes producing hydrogen fluoride by reacting a metal fluoride, such as calcium fluoride, with sulfuric acid. U.S. Pat. No. 3,755,532 describes reacting fluosilicic acid with sodium carbonate to form a sodium fluoride solution, which can then be acidified with sulfuric acid. U.S. Pat. No. 3,278,265 discloses reacting a metal fluoride with a gaseous mixture containing sulfur dioxide to produce hydrogen fluoride.
Therefore, there is a need for a simple, high-yield process for recovering high purity hydrogen fluoride from phosphoric acid plant process streams.